This invention relates generally to water cooled chillers and, more specifically, to the interconnection of two vapor compression refrigeration systems in a series-counterflow arrangement.
Water cooled chillers in a series-counterflow arrangement consist of two independent vapor compression refrigeration systems with chilled water and condenser water circuits that are common to both circuits and are arranged in series. This arrangement allows for an increased coefficient of performance (COP) over a single refrigeration circuit design because the separate circuits with series counterflow have a lower average pressure differential between the evaporator and condenser, thus requiring less energy to compress refrigerant from the evaporator to the condenser.
In such a system, water in each of the evaporators and the condensers flows through a plurality of tubes that span both refrigeration circuits, with the refrigeration circuits being separated by a tubesheet which is located at the middle of the tubes, and with each tube being hermetically sealed to the tubesheet, typically by expansion of the tube to the tubesheet.
One problem that arises is that of servicing the tubes such as may be required if a tube fails in operation. Such removal of a tube requires cutting the tube at all locations where it has been expanded and then pulling the tube out. It is not possible to completely remove a tube since there is no access to cut the tube at the center tubesheet location, which is inside the refrigerant boundary. If a tube is cut internally, or if a tube fails in operation, a leak path is created between the circuits that does not allow for operation of either circuit, thus adversely impacting both reliability and serviceability.
Another problem with a dual circuit system is that of control. A critical parameter for control of a water cooled chiller is the use of the leaving temperature differential, which is the difference in the temperature of the water leaving a heat exchanger and the refrigerant temperature within the heat exchanger. Since the water tubes span both refrigerant circuits in a dual system, it is not possible to obtain the leaving water temperatures of the upstream circuit's condenser or evaporator.
In addition to serviceability and control as discussed hereinabove, prior art heat exchanger tubes that span dual circuits pose problems of reliability, accessibility, shipping and performance. That is, because the common tubes extend across both circuits, it is impossible to optimize the heat transfer tubes in each circuit independently, and shipping of machines that are longer due to the longer tubes can be difficult.
It is desirable to have a two water pass arrangement, wherein entering and leaving water connections can be made from the same location on the chiller, thus allowing access to a tubesheet of the cooler and condenser on the non-connection end without requiring removal of the water piping to the chiller for cleaning or replacing tubes. Also, for those skilled in the art, a two pass arrangement can be desirable for obtaining higher water velocities in the heat exchanger tubes while maintaining a fixed number of heat exchanger tubes. This invention allows for two pass heat exchangers with a series counterflow arrangement by way of a novel machine arrangement and waterbox design.